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With the urgent need to respect the ENVIRONMENT, the use of Flue Gas Desulphurisation systems (FGD) WORLDWIDE is rapidly becoming mandatory. CSM has now designed and built eight stacks - with a total of 18 flues 2.3M to 7.3M A specifically for WET PROCESS FGD systems with temperatures ranging from 40 deg C to a refinery stack operating from 80 deg C to 300 deg C. The materials for construction of FGD systems, their ductwork and chimney flues are required to survive the cleaner but aggressive water laden low temperature gases after wet FGD treatment or potentially abrasive low temperature gases from dry FGD system. The term gas is used here liberally. There are three basic states all relative to temperatures: Liquid occurs when the gas temperature is below its dew point. Vapour occurs when the gas is below its gaseous state temperature. Gas is when the temperature is high enough to keep it so.
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Each gas mixture has a different Dew and Vapour point temperature, such it is necessary to calculate the transition temperatures. Where it is found the gas will fall below its permanent gaseous state temperature, then suitable protection will be needed to prevent corrosion of the liner and entry ductwork. The choice of liner protection depends on several factors, but the prime is usually cost as corrosion protection is an expensive item. Secondly if the work is retrofit, the existing liner material will limit the choice. Wet FGD flue gas contains around 10% water with dissolved gases at a ph of 2 - 3. The volume of water deposited in the chimney is large requiring a proper water drainage system to remove the acid water. SS317 or nickel alloy pipe work is required as PVC and similar products do not withstand the often not acid water. The
water used in FGD system may be "fresh" or seawater. Where
high chloride contents are present. Flues and Ductwork Materials for Corrosion Protection The structural elements of flues and ductwork are generally constructed from: --Steel,
alloy metal, clad, wallpapered steel plates or coated steel
--Acid
resisting brickwork
--Fibreglass
Acid Resisting Brickwork - the best system for long term next to nil maintenance is an Alloy clad or wallpaper metal liner. Dense Clay Interlocking bricks (to ASTM 279 Class 3) ensure a long term leak proof liner. Brickwork can be free standing for the full height or built floor to floor. The brickwork is insulated externally. Entry ducts are usually of C276 clad solid or wallpapered metal. Comments: Acid brickwork is long proven in harsh gaseous environments, however its weakness is temperature cycling (double shifting operations or turning off the FGD system at nights). If the operating temperature is reasonably even or increases and decreases in temperature are smooth then a brick flue will last for 15 years plus. Water resistant mortars should be used where flue gases are in dew or vapour forming temperatures. Interlocking brickwork although slightly more expensive is structurally and gas tight far superior to standard bricks. This system is not suitable in areas of high seismic activity. METAL LINERS Commonly used materials and their applications are as follows:
Comments: These materials have been in use for many years. SS317L now has a track record of ph3 10% H2O gas of over 10 years. C276 is the most commonly used material in either wallpaper or clad form. NACE (National Association of Corrosion Engineers) standard has been used for wallpaper. Caution is needed for stacks subject to deflection (sway) from wind and earthquake. The wallpaper attachment welds need to be designed as the NACE standard recommendations are inadequate for chimneys subject to deflection. Liner skin tension and compressive forces are high when the liner is subject to chimney windshield deflection and only Dynamic Analysis will reveal the potential magnitude of this forces. FGD wet scrubbers an average limit gases to around ph3 in with large volumes of acid water. Water retained in crevices (or welds) may concentrate to ph2 or even 1. Hence it is essential to use high grade materials. If seawater is used Ti2 is the preferable material to use. Coated Steel Glass Flake Vinyl Esters Comments: The
track record of coatings in major ductwork and flue systems is not proven.
New materials used in oil pipelines are now becoming available. They
lack chimney flue track record but survive well is not aggressive pipelines
application. Manufacturers are reluctant to give long-term guarantees.
Inspections and maintenance of coating systems are required at approximately
two-year intervals. There is significant initial cost saving using a
coating, but it is believed future maintenance works will significantly
eat into cost savings. Internal Blocks Borosilicate foamed glass blocks fixed with adhesive to the inside face of liners/flues or concrete walls. Comments: The product is an impermeable insulating acid and alkali resistant lightweight foamed borosilicate glass block of approx 30 - 50mm thickness, stuck on with urethane based adhesives. The adhesive (urethane based) used to stick it to steel or other surfaces is highly flammable with Flash points between 15 - 40 deg C. Post installation modifications to steel liners using oxy acetylene cutting or the like pose major fire risks. The foamed glass block is brittle and may easily be squashed/broken in one's hand. Hence we understand areas of traffic or shard bands/corners are protected by other materials - eg C276 or acid resisting cements or other materials. A
web search reveals there appears to be only one manufacturer leading
to a lack of open market price negotiation. Fibreglass (FRP) Liners These are liner tubes made from structural fibreglass with embedded steel stiffners. Their use to date is limited as generally they are very expensive due to the limited number of manufacturers with such experience. Fibreglass can withstand temperatures up to about 160 deg C. However it does not perform well if vibration/turbulence is present in the gas. This in particular affects the entry area. In
the USA FRP have been marking steady progress with 2 - 3 major designers
and manufacturers. They are much cheaper than Alloy metals, but are
susceptible to damage from vibration. Hence 8 chimneys with Turbulent
gas flows, or large deflections due to wind or earthquake are generally
not suitable for FRP. |
